Impacts of poultry manure and biochar amendments on the nutrients in sweet potato leaves and the minerals in the storage roots

Poultry manure (PM) has demonstrated its potential to enhance crop nutritional quality. Nevertheless, there remains a dearth of knowledge regarding its synergistic effects when combined with wood biochar (B) on the nutrient concentrations in sweet potato leaves (Ipomoea batatas L.) and the mineral content stored in sweet potato storage roots. Hence, a two-year field trial was undertaken during the 2019 and 2020 cropping seasons in southwestern Nigeria, spanning two locations (Owo—site A and Obasooto—site B), to jointly apply poultry manure and wood biochar as soil amendments aimed at enhancing the nutritional quality of sweet potato crop. Each year, the experiment involved different combinations of poultry manure at rates of 0, 5.0, and 10.0 t ha−1 and biochar at rates of 0, 10.0, 20.0, and 30.0 t ha−1, organized in a 3 × 4 factorial layout. The results of the present study demonstrated that the individual application of poultry manure (PM), biochar (B), or their combination had a significant positive impact on the nutrient composition of sweet potato leaves and minerals stored in the sweet potato storage roots, with notable synergistic effects between poultry manure and biochar (PM × B) in enhancing these parameters. This highlights the potential of biochar to enhance the efficiency of poultry manure utilization and improve nutrient utilization from poultry manure. The highest application rate of poultry manure at 10.0 t ha−1 and biochar at 30.0 t ha−1 (PM10 + B30), resulted in the highest leaf nutrient concentrations and mineral composition compared to other treatments at both sites. Averaged over two years, the highest application rate of poultry manure at 10.0 t ha−1 and biochar at 30.0 t ha−1 (PM10 + B30) significantly increased sweet potato leaf nutrient concentrations: nitrogen by 88.2%, phosphorus by 416.7%, potassium by 123.8%, calcium by 927.3%, and magnesium by 333.3%, compared to those in the control (PM0 + B0). The same treatment increased the concentration of sweet potato root storage minerals: phosphorus by 152.5%, potassium by 77.4%, calcium by 205.5%, magnesium by 294.6%, iron by 268.4%, zinc by 228.6%, and sodium by 433.3%, compared to the control. The highest application rate of poultry manure at 10.0 t ha−1 and biochar at 30.0 t ha−1 yielded the highest economic profitability in terms of gross margin (44,034 US$ ha−1), net return (30,038 US$ ha−1) and return rate or value-to-cost ratio (VCR) (263). The results suggested that the application of poultry manure at 10 t ha−1 and biochar at 30 t ha−1 is economically profitable in the study areas and under similar agroecological zones and soil conditions.

The initial soil physical and chemical properties (0-15 cm depth) at site A and site B prior to experimentation in 2019 indicated low levels of organic carbon, total nitrogen, available phosphorus, exchangeable potassium, calcium, and magnesium (Table 1).The annual rainfall totals for 2019 and 2020 were 1093 and 1154 mm, respectively.The total evaporation amounts were 1332 mm in 2019 and 1310 mm in 2020, while the average air temperature was 29.0 °C in 2019 and 29.3 °C in 2020 (Fig. 1).In terms of chemical characteristics, poultry manure was found to be slightly acidic and exhibited elevated levels of nitrogen (N), phosphorus (P), and various micronutrients in comparison to biochar.Conversely, biochar displayed slightly alkaline properties and had higher concentrations of organic carbon (OC), potassium (K), calcium (Ca), and magnesium (Mg), and C:N ratios than poultry manure (Table 2).

Impact of year, site, poultry manure and biochar and their combination on soil chemical properties
The effects of site, poultry manure and biochar on soil chemical properties in 2019 and 2020 are shown in Table 3, and Supplementary Material Figure S1 and Figure S2.In both years (2019 and 2020), site, poultry manure and biochar played a significant role in influencing the soil chemical properties when studied as individual factors.In both years (2019 and 2020), poultry manure and biochar applied alone significantly increased the soil pH, organic carbon (OC), total nitrogen (TN) (Table 3, Supplementary Material Figure S1), available phosphorus (P), exchangeable (K), exchangeable (Ca) and exchangeable (Mg) (Table 3, Supplementary Material Figure S2), and the concentration increased with increasing poultry manure and biochar application rates.Poultry manure increased the soil pH, OC, TN (Table 3, Supplementary Material Figure S1), P, K, Ca and Mg (Table 3, Supplementary Material Figure S2) at rate ranging from 0 to 10.0 t ha −1 .Similarly, biochar increased the soil pH, OC, TN (Table 3, Supplementary Material Figure S1), P, K, Ca and Mg (Table 3, Supplementary Material Figure S2) at rate ranging from 0 to 30.0 t ha −1 .The application rate of poultry manure at 10.0 t ha −1 + biochar at 30.0 t ha −1 (PM10 + B30) had the greatest effect on the soil chemical properties among all the treatments.The control www.nature.com/scientificreports/(no application of poultry manure or biochar) had the lowest values of soil chemical properties.The second year (2020) had higher concentrations of soil pH, OC, TN, P, K, Ca and Mg than the first year (2019) (Table 3).
When examined as individual factors, year (Y) and site (S) exerted noticeable influences on the concentrations of soil pH, OC, N, P, K, Ca and Mg (Table 3).When studied as an individual factor, poultry manure significantly (p = 0.05) increased the concentrations of soil pH, OC, N, P, K, Ca and Mg.Similarly, the application of biochar as an individual factor, significantly (p = 0.05) increased the soil chemical properties (Table 3).Furthermore, Table 1.Mean ± standard deviation of soil physical and chemical properties (0-15 cm depth) of the site A and site B prior to experimentation in 2019; Low: nutrient content value below the critical level recommended for crop production; High: nutrient content value above the critical level recommended for crop production.Three replicates were used for the analysis in the table.the combined effects of various interactions-year and site (Y × S), year and poultry manure (Y × PM), year and biochar (Y × B), poultry manure and site (PM × S), biochar and site (B × S), and poultry manure and biochar (PM × B)-exhibited significant influences on soil pH, OC, N, P, K, Ca and Mg.When all four factors-year, site, poultry manure and biochar (Y × S × PM × B) were considered together, the interactions were significant for soil pH, OC, N, K, Ca and Mg, but the interactions were not significant for soil P (Table 3).

Impact of year, site, poultry manure and biochar and their combination on the nutrient content of sweet potato leaves
Table 4, and Supplementary Material Figure S3 and Figure S4 illustrate the impacts of year, site, poultry manure and biochar on the nutrient concentrations in sweet potato leaves in 2019 and 2020.In both years, the interplay of site conditions, poultry manure, and biochar had significant effects on the nutrient concentrations in sweet potato leaves.Specifically, the application of either poultry manure or biochar, independently, led to a notable increase in the concentrations of N and P (Table 4, Supplementary Material Figure S3) as well as K, Ca, and Mg (Table 4, Supplementary Material Figure S4).Moreover, the concentration increased in a direct correlation with the quantity of poultry manure and biochar applied.In both 2019 and 2020, the nutrient concentrations in sweet potato leaves exhibited elevated levels of N and P (Table 4, Supplementary Material Figure S3) as well as K, Ca, and Mg (Table 4, Supplementary Material Figure S4) when poultry manure was applied at rates ranging from 0 to 10.0 t ha −1 .Similarly, the application of biochar, within the range of 0 to 30.0 t ha −1 , led to increased concentrations of N and P (Table 4, Supplementary Material Figure S3) as well as K, Ca, and Mg (Table 4, Supplementary Material Figure S4) in sweet potato leaves.Compared with those in the control group (no poultry manure or biochar application), the use of poultry manure, biochar in isolation, or their combined application at varying rates, resulted in significant increases in the levels of N, P, K, Ca, and Mg in sweet potato leaves, during both years.Notably, the coapplication of poultry manure and biochar at different levels yielded the highest leaf concentrations of N and P (Table 4, Supplementary Material Figure S3) as well as K, Ca, and Mg (Table 4, Supplementary Material Figure S4) in sweet potato, compared to the application of poultry manure or biochar individually.The peak concentrations of N, P, K, Ca, and Mg in sweet potato leaves were achieved when the highest application rates of both amendments were used (poultry manure at 10.0 t ha −1 in combination with biochar at 30.0 t ha −1 ) in both years, denoted as (PM10 + B30).When studied as individual factors, both year (Y) and site (S) had a significant influence (P < 0.05) on the levels of leaf N, P, K, Ca and Mg in sweet potato plants (Table 4).Similarly, when examined as an individual factor, the application of poultry manure (PM) significantly (P < 0.05) increased the concentrations of leaf N, P, K, Ca, and Mg.Similarly, the application of biochar (B) as an individual factor, significantly (P < 0.05) increased the leaf nutrient concentrations of sweet potato plants (Table 4).Furthermore, the interactive effects of factors such as year and site (Y × S), year and poultry manure (Y × PM), year and biochar (Y × B), poultry manure and site (PM × S), biochar and site (B × S), and poultry manure and biochar (PM × B)-had significance impacts on leaf N, P, K, Ca, and Mg concentrations in sweet potato plants.Notably when all four factors-year, site, poultry manure and biochar (Y × S × PM × B) were considered together, their interactions remained significant (Table 4).

Impact of year, site, poultry manure and biochar and their combination on the minerals accumulated in sweet potato storage roots
In both 2019 and 2020, site and the application of poultry manure and biochar had a significant impacts on the mineral content of sweet potato storage roots.In general, whether applied individually or in various www.nature.com/scientificreports/combinations, poultry manure, biochar, or their mixtures resulted in a significant (P < 0.05) increase in the concentrations of minerals such as P (Table 5, Supplementary Material Figure S5), K, Ca, Mg (Table 5, Supplementary Material Figure S6), Fe, Zn, and Na (Table 5, Supplementary Material Figure S7) in sweet potato storage roots, compared to those in the control group, which received no poultry manure or biochar in both years.In both years, the application of poultry manure led to a significant (P < 0.05) increase in the concentrations of P (Table 5, Supplementary Material Figure S5), K, Ca, Mg (Table 5, Supplementary Material Figure S6), Fe, Zn, and Na (Table 5, Supplementary Material Figure S7) in sweet potato storage roots, with application rates ranging from 0 to 10.0 t ha −1 .Similarly, biochar application significantly (P < 0.05) increased the concentrations of P (Table 5, Supplementary Material Figure S5), K, Ca, Mg (Table 5, Supplementary Material Figure S6), Fe, Zn, and Na (Table 5, Supplementary Material Figure S7) in sweet potato storage roots, with application rates ranging from 0 to 30.0 t ha −1 .When poultry manure and biochar were combined at various levels, even higher concentrations of P (Table 5, Supplementary Material Figure S5), K, Ca, Mg (Table 5, Supplementary Material Figure S6), Fe, Zn, and Na (Table 5, Supplementary Material Figure S7) were detected in sweet potato storage roots than when poultry manure or biochar was applied alone.The highest mineral values for P, K, Ca, Mg, Fe, Zn, and Na were observed when poultry manure was applied at 10.0 t ha −1 + biochar at 30 t ha −1 (PM10 + B30), surpassing the other treatments.The control group displayed the lowest mineral nutrient levels in sweet potato storage roots in both years.When examined as an individual factor, year (Y) and site (S) exerted noticeable influences on the concentrations of P, K, Ca, Mg, Fe, Zn, and Na in the storage roots of sweet potato plants (Table 5).When studied as individual factors, the application of poultry manure (PM) and biochar (B) also affected mineral nutrition in sweet potato storage roots.Furthermore, the combined effects of various interactions-year and site (Y × S), year and poultry manure (Y × PM), year and biochar (Y × B), poultry manure and site (PM × S), biochar and site (B × S), and poultry manure and biochar (PM × B)-exhibited significant influences on all mineral nutrients in the sweet potato storage roots (Table 5).When all four factors-year, site, poultry manure and biochar (Y × S × PM × B) were considered together, their interactions remained significant (Table 5).

Discussion
The low levels of organic carbon, total nitrogen, available phosphorus, exchangeable potassium, calcium, and magnesium, high bulk density and low pH observed at both site A and site B prior to experimentation in 2019 (Table 1), could be attributed to soil degradation due to continuous cropping practices, suggesting significant soil fertility challenges in the region.The slightly acidic nature of poultry manure aligns with previous studies on the chemical composition of manure from various sources 10,11 .This acidity may influence soil pH when used as a fertilizer, affecting nutrient availability and microbial activity 12 .Additionally, the elevated levels of nitrogen and phosphorus in poultry manure are consistent with the nutrient-rich nature of animal-derived organic materials, suggesting its potential as an effective fertilizer for promoting plant growth 13 .In contrast, the alkaline properties of biochar have been associated with its potential to improve soil pH, soil structure, nutrient retention and Table 3. Impact of year, site, poultry manure and biochar, and their combination on soil pH, organic carbon (OC), total nitrogen (TN), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg).*Significant difference at P < 0.05.www.nature.com/scientificreports/cation exchange capacity 14 .The higher concentrations of organic carbon and essential nutrients such as potassium, calcium, and magnesium in biochar are indicative of its ability to enhance soil fertility and promote plant development 15,16 .The observed difference in the carbon-to-nitrogen ratio between poultry manure and biochar is noteworthy, because it influences decomposition dynamics and nutrient release in the soil 17 .A higher C:N ratio in biochar suggests slower decomposition rates, potentially leading to prolonged nutrient availability in the soil compared to poultry manure 18 .The higher C:N ratio of biochar suggests its potential to act as a stable, longlasting carbon source in the soil, aiding in carbon sequestration and enhancing soil organic matter content 16 .The contrasting chemical characteristics of poultry manure and biochar highlight their distinct roles and potential benefits in agricultural practices.The strategic integration of these organic amendments based on their unique properties can contribute to sustainable soil management and crop productivity.
The second year (2020) had higher concentrations of soil pH, OC, TN, P, K, Ca and Mg than the first year (2019) due to the cumulative and residual effects of repeated applications of poultry manure and biochar.These amendments not only improved soil fertility in the first year but also continued to decompose and release nutrients over time, leading to enhanced nutrient retention and availability.Additionally, biochar has a persistent nature, which means its benefits compound over time, further improving soil properties in the subsequent year.The organic matter from the poultry manure decomposed and further enriched the soil, leading to greater nutrient concentrations in the second year.
The findings presented in this study shed light on the intricate dynamics of nutrient and mineral accumulation in sweet potato plants, particularly in their leaves and storage roots, under the influence of various factors such as year, site conditions, and the application of poultry manure and biochar.The comprehensive analysis of these factors underscores their individual and interactive roles in shaping the nutritional profile of sweet potato plants.The results demonstrated that both the year and site significantly impacted the nutrient concentrations in sweet potato leaves and storage roots due to the complex interaction of environmental, soil and climatic factors that vary by location and over time.This underscores the importance of considering site-specific and temporal factors in soil fertility management.These findings are consistent with previous studies highlighting the sensitivity of crop nutrient uptake to environmental variations and site-specific factors 19,20 .
The findings presented in this study demonstrated a significant impact of poultry manure and biochar on the nutrient content of sweet potato leaves during 2019 and 2020.The application of poultry manure and biochar, either individually or in combination, led to notable increases in the concentrations of N, P, K, Ca, and Mg in sweet potato leaves.This finding aligns with existing research indicating that organic amendments, such as poultry manure and biochar, can enhance nutrient concentrations in plant tissues 21,22 .Averaged over two years, the application of the highest rate of poultry manure at 10.0 t ha −1 and biochar at 30.0 t ha −1 (PM10 + B30) significantly increased sweet potato leaf nutrient concentrations: nitrogen by 88.2%, phosphorus by 416.7%, potassium by 123.8%, calcium by 927.3%, and magnesium by 333.3%; compared to those in the control (PM10 + B30).The observed increase in N, P, K, Ca, and Mg levels in sweet potato leaves corresponds with previous studies highlighting the positive effects of poultry manure and biochar on nutrient availability in soils 23,24 .Gune et al 25 reported that the synergistic effect of application of poultry manure and biochar resulted in the highest N, P and K concentrations in lettuce plants, which is consistent with the concept of nutrient complementarity between organic amendments.This underscores the potential of integrated nutrient management strategies for sustainable agriculture, emphasizing the importance of combining poultry manure and biochar to maximize nutrient benefits in nutrient-deficient soils 26 .
Furthermore, the mineral content in sweet potato storage roots was significantly influenced by year, site conditions, and the application of poultry manure and biochar.The results indicated that the addition of poultry manure and biochar, either alone or in combination, resulted in increased concentrations of essential minerals such as phosphorus, potassium, calcium, magnesium, iron, zinc, and sodium in sweet potato storage roots.The highest mineral values were achieved when poultry manure and biochar were combined at optimal rates, suggesting the potential for integrated nutrient management strategies to maximize nutritional quality.Pooled over two years, the application of the highest rate of poultry manure at 10.0 t ha −1 and biochar at 30.0 t ha −1 (PM10 + B30) increased the concentration of sweet potato root storage minerals: phosphorus by 152.5%, potassium by 77.4%, calcium by 205.5%, magnesium by 294.6%, iron by 268.4%, zinc by 228.6%, and sodium by 433.3%; compared to those in the control (PM10 + B30).This finding underscores the role of organic amendments in enhancing the nutritional quality of sweet potato roots 27 .Table 4. Impact of year, site, poultry manure and biochar, and their combination on leaf nitrogen (N), phosphorus (P), potassium (K), calcium (Ca) and magnesium (Mg) concentrations in sweet potato.*Significant difference at P < 0.05.www.nature.com/scientificreports/Moreover, the synergistic effects of combining poultry manure and biochar at various levels resulted in even greater enhancements in nutrient and mineral concentrations in sweet potato plants compared to their individual applications.This highlights the potential benefits of integrating multiple soil amendments to optimize nutrient management strategies in agricultural systems 28,29 .In addition, the interactive effects of factors such as year and site, year and amendments (poultry manure or biochar), and the combined application of poultry manure and biochar were found to notably affect the nutrient contents in sweet potato leaves and the mineral composition in the storage roots.These interactive effects of various factors on mineral concentrations in sweet potato storage roots further emphasize the complexity of nutrient dynamics in soil-plant systems.These interactions highlight the need for integrated management strategies that consider multiple factors to optimize nutrient uptake and crop productivity 30,31 .This suggests that synergistic interactions between environmental conditions and amendments can influence nutrient uptake by sweet potato plants.These findings are supported by studies highlighting the complex interactions between soil properties, amendments, and plant nutrition 32,33 .
The increase in concentrations of N, P, K, Ca, and Mg in sweet potato leaves, as well as mineral P, K, Ca, Mg, Fe, Zn and Na in sweet potato storage roots, due to the application of poultry manure and biochar, can be Table 5. Impact of year, site, poultry manure and biochar, and their combination on mineral phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn) and sodium (Na) of sweet potato tuber.*Significant difference at P = 0.05.Table 6.Economics of producing sweet potato under each level of poultry manure and biochar tested in 2019 and 2020.In the year of 2019, the price of fresh storage root yield of sweet potato was US$ 1.23 kg −1 .In the year of 2020, the price of fresh storage root yield of sweet potato was US$ 1.23 kg −1 .US$1 = N360 in 2019.US$1 = N370 in 2020.www.nature.com/scientificreports/attributed to the increased availability of macronutrients.This increased availability is essential for soil fertility, as previously highlighted by Liu et al. 34 .The application of poultry manure and biochar improved soil nutrient availability, leading to greater nutrient uptake by sweet potato plants.The enhanced nutrient concentrations in the leaves and the increased mineral content in the storage roots of sweet potatoes, resulting from these applications, can be linked to improved soil quality, the release of nutrients into the soil solution, enhancements in chemical properties, the presence of beneficial organisms, and a more balanced nutritional status for the plants 4,21,[24][25][26] .The observed increase in nutrient concentrations in sweet potato leaves and the minerals stored in sweet potato storage roots can be attributed to the synergistic effects of poultry manure and biochar 4,29 .Poultry manure supplies essential macronutrients and micronutrients crucial for plant growth and development, as highlighted by Kingery et al. 12 .Biochar, on the other hand, enhances soil fertility and nutrient availability due to its porous structure, high cation exchange capacity, and ability to retain water and nutrients 30,32 .The combined application of poultry manure and biochar creates an optimal soil environment that promotes nutrient uptake and translocation within sweet potato plants, in line with the findings of Liu et al. 34 .Recent studies align with findings on the impact of poultry manure and biochar on sweet potato storage roots, supporting the significant increase in mineral concentrations observed.According to Alomari et al. 35 , the application of poultry manure and biochar, either separately or in combination, resulted in a substantial increase in phosphorus, potassium, calcium, magnesium, and sodium contents in lettuce crops.This outcome underscores the effectiveness of this combined amendment in alleviating nutrient deficiencies and enhancing plant nutrition in degraded soils, due to improved nutrient availability.However, a dissenting view was presented by Ayito et al. (2024) 36 , who argued that while poultry manure and biochar applications had positive effects on the mineral content, the efficacy varied based on the soil type and climatic conditions.This dissent emphasizes the need for a nuanced understanding of the contextual factors influencing the outcomes of such agricultural practices.
All treatments involving poultry manure and biochar, irrespective of the application rate, generated higher net profits compared to the control.The most notable increase in profitability was seen with the treatment PM10 + B30, which not only provided the highest sweet potato yield and quality but also demonstrated the most cost-effective approach for sweet potato production.The substantial net returns and high value-to-cost ratio underscore the financial benefits of using higher rates of poultry manure and biochar in sweet potato cultivation in severely degraded soils.

Conclusion
The application of poultry manure, biochar, and their combination to degraded sand and sandy loam soils led to increased nutrient concentrations in sweet potato leaves and minerals stored in the storage roots.This enhancement is attributed to synergistic interactions between poultry manure and biochar.Among the various application rates tested, the most effective combination was found to be poultry manure at 10.0 t ha −1 and biochar at 30.0 t ha −1 .This specific combination not only increased the nutritional quality of sweet potato but also maximized its economic returns.Hence, this application rate is recommended for soil fertility management, ensuring nutritional sustainability, and optimizing sweet potato performance in the forest savanna transition zone of southwest Nigeria.Enriching soil with poultry manure and biochar holds promise for enhancing nutrient concentrations in sweet potato leaves and mineral levels in storage roots, with potential implications for sustainable agriculture, soil restoration, and crop nutrition in degraded environments.However, further research is needed to validate these findings across diverse soil types, crops, agroecological zones, and land-use patterns due to spatial heterogeneity.Long-term field studies focusing on biochar's persistence in soils and its impact on crops are recommended.Additionally, exploring various application rates of poultry manure and biochar, as well as their combined use on soils and crops, is essential for future research.

Study area and treatments
Field experiments were carried out in Owo, Ondo State, Nigeria, during the cropping seasons of 2019 and 2020 from April to August each year.These experiments were conducted at two sites, namely Rufus Giwa Polytechnic (site A) and Obasooto village (site B).Owo is situated in the transition zone between the forest and savanna, with geographic coordinates of approximately 7°12'N latitude and 5°35'E longitude, at an elevation of 348 m above sea level.The soil in Owo belongs to the Okemesi Series 37  (Smyth and Montgomery, 1962) and is classified as an Alfisol (Oxic Tropuldalf) 38  (Soil Survey Staff, 2014) or Luvisol 39 (IUSS Working Group WRB, 2015).This soil type originates from quartzite, gneiss, and schist.The average annual rainfall in this region is approximately 1400 mm, while the mean annual temperature is approximately 32 °C.The sites had been left fallow for a year following rotational arable cropping, and no fertilizer, manure or biochar treatment had been applied to any of the sites in the previous six years.Historical soil management practices included conventional tillage methods such as ploughing, harrowing, and ridging.
Poultry manure (PM) at rates of 0, 5.0, and 10.0 t ha −1 ), along with varying levels of biochar (B) at 0, 10.0, 20.0, and 30.0 t ha −1 , were combined in a 3 × 4 factorial layout.These twelve treatments were arranged in a randomized complete block design, with three replications.Each block consisted of 12 plots, each measuring 5 × 4 m.The blocks were spaced 1 m apart, and the plots were spaced 0.5 m apart.Crop establishment activities were carried out annually in April, and the same site was used consistently throughout the two-year study period.

Land preparation, incorporation of poultry manure and biochar and planting of sweet potato vines
The experimental sites were prepared by slashing the vegetation with a cutlass followed by removing the weeds present.The trial plots were then laid out in accordance with the designated size of 5 × 4 m.The soils were then Vol:.( 1234567890 www.nature.com/scientificreports/tilled to a depth of 20 cm with a handheld hoe.Poultry manure (PM) and biochar (B) were weighed and evenly spread over the soil at the specified rates (PM: 0, 5.0 and 10.0 t ha −1 ; B: 0, 10.0, 20.0 and 30.0 t ha −1 ).These soil amendments were incorporated into the soil to a depth of approximately 10 cm using a handheld hoe, two weeks prior to planting the sweet potato vines.This procedure was repeated in the second year of the experiment.Following soil preparation, sweet potato (Ipomoea batatas L. local variety) vines approximately 40 cm long, were planted in April of each experimental year.One sweet potato vine was planted per hole at a spacing of 1 m × 1 m, resulting in a sweet potato population of 20 plants per plot and 10,000 plants per hectare.The field plots were manually weeded twice, at 3 and 8 weeks after planting (WAP), to prevent weeds from competing with the crops for nutrients, water, and sunlight.During the trial, no irrigation water was applied.Chemical fertilizer or manure or biochar were also not applied during the field experiment.

Soil analysis
Prior to the experiment in 2019, soil samples were taken from a depth of 0-15 cm at 10 randomly selected points across the experimental site.During the harvest in 2019 and 2020, additional disturbed soil samples were randomly collected from the center of each plot at five points per plot, also at a depth of 0-15 cm.All collected soil samples were bulked, air-dried, and sieved through a 2-mm sieve for routine chemical analysis, following the methods described by Carter and Gregorich (2007) 40 .Particle-size analysis was determined using the hydrometer method, and the textural class was determined using a textural triangle.Soil pH was measured in a soil/ water (1:2) suspension with a digital electronic pH meter.The soil organic carbon content was determined using the Walkley-Black procedure involving dichromate wet oxidation.Total nitrogen (N) content was measured using micro-Kjeldahl digestion and distillation techniques.Available phosphorus (P) content was determined by Bray-1 extraction followed by molybdenum blue colorimetry.Exchangeable potassium (K), calcium (Ca), and magnesium (Mg) were extracted with a 1 M ammonium acetate (NH4OAc) solution at pH 7. Exchangeable K was analyzed using a flame photometer, while exchangeable Ca and Mg were measured with an atomic absorption spectrophotometer.

Poultry manure and biochar preparation and analysis
The poultry manure (PM) used in this study was sourced from the poultry unit of the Teaching and Research Farm at Rufus Giwa Polytechnic, Owo, Ondo State.To facilitate mineralization, the poultry manure was subjected to a composting process lasting three weeks.The biochar materials employed in the experiments were derived from hardwood sources, including Parkis biglosa, Khaya senegalensis, Prosopis africana, and Terminalia glaucescens.These biochar materials were acquired from a local charcoal producer in Owo, Ondo State, Nigeria, who traditionally produces charcoal for domestic use.The carbonization process for the biochar was monitored using a thermocouple thermometer, and the kiln maintained an average temperature of 580 °C after 24 h.Both poultry manure and biochar were selected as soil amendments due to their wide availability and sustainability within the region.Small subsamples of approximately 5 g each of the processed poultry manure and biochar materials used in the experiments were subjected to analysis to determine their nutrient compositions.Prior to analysis, both the poultry manure and biochar samples were prepared by air-drying, crushing, and sieving through a 2-mm sieve.These subsamples underwent various analyses, including measurements of electrical conductivity (EC), pH, ash content, organic carbon (OC), total nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg), as well as the concentrations of trace elements such as copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), and sodium (Na).
The EC and pH of both the poultry manure and biochar were determined in a 1% (w/v) suspension in deionized water prepared by shaking at 100 rpm for 2 hours 41 .The ash content, expressed on a dry weight basis, was determined by combusting poultry manure and biochar samples in a muffle furnace at 750 °C for 6 h, following the ASTM D1762-84 (2021) 42 standard.The OC content and total N, P, K, Ca, and Mg were determined using established procedures, such as the Walkley and Black method for OC content and micro-Kjeldahl digestion for total N. Other nutrients, including P, K, Ca, and Mg, were determined through wet digestion using a mixture of HNO 3 -H 2 SO 4 -HClO 4 acids.Specifically, P was extracted using a Bray-1 solution and analyzed by molybdenum blue colorimetry, while exchangeable K, Ca, and Mg were extracted using a 1 M ammonium acetate, pH 7 solution.The exchangeable K was measured with a flame photometer, and Ca and Mg were determined using an atomic absorption spectrophotometer.To determine the concentrations of trace elements, such as Cu, Fe, Mn, Zn, and Na, samples of poultry manure and biochar with known quantities were incinerated at 760 °C in a muffle furnace.The resulting ash was treated with HCl, diluted with deionized water, and then analyzed for trace element concentrations.Cu, Fe, Mn, and Zn levels were determined in both poultry manure and biochar samples through the use of an atomic absorption spectrophotometer.The concentration of Na in these samples was measured using a flame photometer.

Analysis of sweet potato leaves
During the cropping seasons of 2019 and 2020, sweet potato leaves were collected randomly from five plants in each plot for chemical analysis.These leaves, which were in the 2-to 3-week age range, were harvested 90 days after planting.The preparation of the samples involved a series of steps: the samples were initially dried in an oven set at 80 °C for 48 h and subsequently ground using a Willey-mill.To determine the levels of nitrogen (N) in the leaves, the micro-Kjeldahl digestion method was used.In addition to nitrogen analysis, the samples were subjected to a dry ashing process at 500 °C for 6 h in a furnace.A nitric-perchloric-sulphuric acid mixture was used to extract phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg).Phosphorus levels were determined through the colorimetric vanadomolybdate method, while potassium was quantified using a flame www.nature.com/scientificreports/photometer.Calcium and magnesium levels were assessed using the ethylene diamine tetraacetic acid (EDTA) titration method 43 .

Analysis of the mineral composition in sweet potato storage roots
In both years, an examination was conducted to analyze the mineral composition in the storage roots of sweet potato.After a growth period of 5 months, ten central plants from each plot were harvested at the two experimental sites.Subsequently, five storage roots of uniform size were chosen at random, washed, and then peeled.These storage roots were then collectively mixed and chopped into small pieces.The resulting chips were meticulously blended to create a consistent sample of root tissue derived from the five initial storage roots.A 100-g sample was selected and subjected to a 24-h drying process in an oven at 60 °C.Afterward, the dried samples were ground and securely stored in airtight containers for subsequent chemical analysis.To determine the levels of mineral elements, including phosphorus, potassium, calcium, magnesium, iron, and zinc, standard methods endorsed by the Association of Official Analytical Chemists 43 were used.The samples were digested using a mixture of HNO 3 , H 2 SO 4 , and HClO 4 , and the phosphorus content was measured through the molybdenum blue colorimetric technique.Potassium, calcium, magnesium, iron, and zinc contents were assessed via atomic absorption spectrophotometry, following the protocols detailed in AOAC 43 .

Statistical analysis
The experiments followed a randomized complete block design, employing factorial layouts to test the main impacts of year (Y), site (S), poultry manure (PM), and biochar (B), as well as the interactions of Y × S, Y × PM, Y × B, PM × B, PM × S, B × S, and Y × S × PM × B on the nutrients in sweet potato leaves and the minerals in the storage roots.The data analysis utilized a two-way analysis of variance (ANOVA) performed using the SAS (Statistical Analysis System) 44 statistical package version 9.4 and Microsoft Office Excel 2013 software packages.The treatment means were separated using Fisher's least significant difference (LSD) test at a significance level of P < 0.05.

Cost-to-benefit analysis
A cost-to-benefit analysis was performed to determine the relative economic returns of the treatments using 2019 and 2020 annual market prices.The analysis involved calculating the total yield and cost-benefit ratios in US dollars (1$US = N360.00Nigerian currency in the year 2019 and N370.00 in the year 2020), based on the harvest from the central bed (1 m 2 ) within each plot.The costs for farm services were sourced from the Oja Oba market in Owo Local Government Area of Ondo State, Nigeria.

Table 2 .
Chemical composition of poultry manure and biochar used in the experiment.